1. Field of the Invention
The invention relates to a cooktop with a nonmetallic hotplate, in particular a glass ceramic hotplate, which has at least one cooking zone to which an electric heating unit is assigned, and having a device for detecting the presence and/or size of metal pots on the cooking zone, which has a measuring sensor disposed in the region of the cooking zone and an evaluation device communicating with the measuring sensor.
2. Prior Art
A cooktop with a switching device for supplying energy to the heating unit is known from Austrian Patent 238 331. The switching device enables the supply of energy to the heating unit when the pot is put on the stove and interrupts the supply of energy when the pot is removed. Detecting the pot on the cooking surface is done by means of a proximity switch, which is not defined in further detail in this patent.
German Patent Disclosures DE-A 35 33 997 and DE-A 33 27 622 describe pot detection systems with optical sensors. Pot detection systems with inductive sensors are known from German Patent Disclosures DE-A 37 11 589 and DE-A 37 33 108.
The known inductive proximity switches are based on the principle of the damping of an oscillation circuit caused by eddy current losses in metals that are located in the magnetic stray field of the sensor coil. It is a disadvantage that the coil may have many windings to achieve adequate sensitivity. Furthermore, changes in the electrical properties of the coil material, at the high temperatures that occur in the cooking zones of the cooking surface, cause a temperature drift of the measurement signal, which is on the order of magnitude of the signals caused by putting the pots on the stove or taking them away. To avoid measurement errors from temperature changes, it is known to evaluate rates of signal change. In that case, a relatively complicated temperature compensation can be omitted.
European Patent Disclosures EP-A 0 442 275 and EP-A 0 469 189 describe pot detection systems of this kind that have an inductive sensor in the form of a coil that is part of an oscillation circuit. To make it possible to distinguish between the signal change when the pots are put on the stove or taken away from the signal change that can be ascribed to temperature changes, the different rate of signal change is detected, which differs markedly when pots are put on the stove or taken away from the rate of signal change caused by temperature changes. It is disadvantageous, however, that the above systems must be located permanently in the readiness state, because pots can be detected only when they are put on the stove or taken away. Conversely, static pot detection is not possible.
Capacitive sensors for pot detection are known from International Patent Disclosure WO 90/07851 and European Patent Disclosure EP 0 429 120. These sensors are disadvantageous in the sense that only small useful signals, which are hard to evaluate, are obtained, and the systems are vulnerable to electromagnetic factors. Moreover, the measurement signals can be affected by nonmetallic materials, such as the hands of a person, damp cloths, etc.
From U.S. Pat. No. 4,334,135, a pot detection system for an inductively heated glass ceramic cooktop has become known, in which a receiving coil that detects the changes in the magnetic field caused by a pot placed on the stove is disposed above the induction heating coil.
Heating coils for induction devices in general utilize ferritic parts to carry the field; they are disposed below the induction heating coil. If no pot is placed on the stove, the circuit is not closed, and the field must pass through the air. When a pot is placed on the stove, the field is guided in the ferritic material and amplified. The receiving coil, located in the space between the induction heating coil and the pot, is engaged by this amplified field and outputs an amplified signal.
The principle of operation of the known device is the principle of the magnetic circuit closed by the pot and the accordingly increased magnetic flux; that is, in the known case, the signal increases when pot is placed on the stove.
In principle, this effect works only with ferromagnetic pots and pans; that is, it does not work with pots and pans of special steel, aluminum, or copper, but for induction devices this is not a restriction because such devices can be operated only with such pots and pans anyway. The relative permeability of the material comprising the pot is decisive for the function.
The known pot detection system is therefore disadvantageously limited to inductively heated cooking zones.
Another disadvantage of the known case is that two coils connected counter to one another are needed to detect pots and pans that have shifted position. Detecting pots and pans of different sizes to control a two-circuit heating body is not contemplated in the known case.